This invention relates generally to automatically focusing, solid state array cameras, and particularly to an auxiliary focusing circuit for use with such cameras.
The use of automatically focusing cameras is rapidly becoming commonplace in various manufacturing operations. An example of such a use is the measurement of the characteristics of the phosphor screen on the faceplate panel of color television picture tubes. The screen of a color television tube is composed of triads of phosphors which are applied in a repetative sequence. Each of the phosphors emits a different color of light when impacted by electrons. There are two types of phosphor screens. In the first type screen, the phosphors are applied as vertical stripes which extend the full viewing height of the screen. In a matrix version of this screen, the vertical phosphor stripes are separated by a light absorbing material. Another type of screen is a dot screen, in which the phosphors are applied in triads of dots. In a matrix version of this screen type, the dots are separated by the light absorbing material. For both types of screens it is preferable to determine that the dimensions of the phosphor bearing areas are proper after application of the light absorbing matrix, but before application of the phosphor elements. Accordingly, the clear areas between the black matrix materials are measured prior to applying the phosphors to the screen. Also, it is sometimes useful to make measurements after the phosphors are applied.
Typically, measurements are made by shining light through a selected section of the screen and focusing the light with an automatically focusing camera. After the camera is properly focused, the light impinges upon a light responsive imaging medium, such as a photodiode array or a charge coupled device (CCD). The pixels of the imaging medium which are shaded by the black matrix portions of the screen are charged to one level, and the pixels which receive light through the line or dot portions are charged to a different level. The pixel information is then provided to a computer, which controls the system, and used to determine the dimensions of the phosphor bearing areas and of the black matrix portions of the screen.
Typically, during the imaging, light is focused by the camera onto a moveable reflective galvo which is rotated to cause the light to scan lines from the area being measured across the light responsive imaging medium. Light from the galvo is reflected to a fixed mirror and then to the imaging medium so that a large number of data lines are received for each of the measured areas. When the screen is of the line type, the galvo scans the screen in a direction which is perpendicular to the direction of the phosphor lines. The edges of the lines are used for the automatic focusing. When the screen is of the dot type, the dots are used to focus the camera. However, with a dot type of screen, it is possible for the imaging medium to lie between the horizontal rows of dots and no information useful in automatically focusing the camera is imaged onto the imaging medium. When this condition occurs, the camera cannot be focused and no useful data is obtained for that position of the galvo. For these reasons, there is a need for an auxiliary focusing circuit which assures that imaging information is received for all orientations of the rotatable galvo and for all positions on the face of the panel. The present invention fulfills this long felt need.